By Prof Arjun Kumar Asst professor Electrical Engineering BCE B hagalpur Introduction of 1ph Induction Motor A singlephase induction motor comprises a singlephase distributed winding on the stator and ID: 920129
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Slide1
Module-4
Single phase Induction motor
By
Prof. Arjun Kumar
Asst. professor
Electrical Engineering
B.C.E.
B
hagalpur
Slide2Introduction of 1-ph. Induction Motor
A single-phase induction motor comprises a single-phase distributed winding on the stator and
normal
squirrel-cage rotor.
A
vailability of
a wide variety of small-size motors of fractional kilowatt ratings
.
M
otors
are employed in
fans, refrigerators
, mixers, vacuum cleaners; washing machines, other kitchen equipment, tools, small
farming
appliances
, etc
.
B
ehaviour
of
single-phase
induction
motor can be explained on the basis of double-field revolving
theory and cross magnetic field theory.
Single phase induction motors are simple, robust, reliable and cheaper for small ratings. They are available up to 1 KW rating
Slide3CUT SECTION VIEW
Slide4T
ypes
of single phase induction motor
Split phase induction
motor
Capacitor
start
induction motor
Capacitor
start capacitor run induction
motor
Shaded
pole induction
motor
Permanent
split capacitor motor
Slide5Constructional features
Fig.4.1 single phase I.M
Slide6Main parts of
Single ph. I.M.
Single ph. I.M. having
two main
parts:
Stator
Rotor
Stator:
stator
is a stationary part of induction
motor.
A single phase AC supply is given to the stator of single phase induction
motor. In this machine stator having two windings one is main winding and other is auxiliary windings.
Rotor:
rotor
is a rotating part of an induction motor. The rotor connects the mechanical load through the shaft. The rotor in the single-phase induction motor is of squirrel cage rotor
type
.
Slide7Constructional View
Fig.4.2 constructional view of 1-ph.I.M.
Slide8Double revolving field theory
Fig.4.3 single phase I.M
Slide9Figure 4.3 gives the schematic diagram of a single-phase induction motor with one stator winding and a squirrel-cage rotor. The winding is distributed in space so that the space fundamental of
mmf
is the most dominant component of the actual
mmf
distribution. When the winding carries a sinusoidal current, it produces a
sinusoidally
space distributed
mmf
whose peak value pulsates with time. As seen from the axis of the winding, the
mmf
at any angle
is
---------(4.1)
where
is the angle measured from the winding axis.
Now
------(4.2)so that the mmf has both space and time distribution expressed as --------(4.3)This equation can be trigonometrically manipulated into the form ----(4.4)
Equation (4.4) tells us that a pulsating single-phase field can be considered as superposition of two rotating fields rotating at synchronous speed (
elect. rad/s) in opposite directions:
The forward rotating field,
----(4.5)
Backward rotating field,
---(4.6)
Both these fields have an amplitude equal to
where
is the maximum value of the pulsating
mmf
along the axis of the winding. The splitting of a single pulsating field into two rotating fields rotating in opposite directions
T
wo rotating
fields rotating equivalent of a pulsating field
Fig.4.4
Slide12Rotor slip with respect to two rotating field
Fig.4.5
Slide13Fig.
4.5
shows the forward and backward rotating fields along with the rotor which is rotating at speed
in the direction of the forward field.
The slip of the rotor with respect to the forward rotating field
is then
rotor slip with respect to the backward rotating field
is
Torque slip characteristic of single winding 1
ph
I.M
Slide15Equivalent circuit of single
ph
I.M
Slide16The performance of a single-phase induction motor can be obtained by analysis of the circuit model of the motor.
The air-gap powers for the forward and backward fields are given by
Air-gap power for backward field
Here,
main winding current
and
are the real parts of the complex number impedances
and
The torques produced by the two fields can be expressed as
Resultant torque developed is
Rotor copper-loss corresponding to forward field=
Total rotor copper-loss
=
Split phase 1-ph I.M.
Slide20Slide21Pulsating fields
and
can be divided into two balanced sets of opposite phase sequence
=
=
The inverse of the relationships can be expressed as
Split phase 1-ph I.M.
When a motor is provided with two windings, even though these are excited from the same voltage (supply being single-phase), the currents in the two windings can be made out-of-phase by adjustment of the impedance of the auxiliary winding in relation to the main winding. As a result
and
constitute an unbalanced field set with 90° elect. space-phase
relationship.
The two symmetrical components now being unequal
The forward rotating field is made stronger than the backward rotating field resulting in the net production of starting torque.
Resistance split 1
ph
I.M
.
M
otor employs an auxiliary winding with a higher
R
/
X
ratio as compared to the
main
winding.
High
R
/
X
ratio
of auxiliary winding is
achieved by using a smaller number of turns of thin wire for the
auxiliary
winding.This difference in the R/X ratio causes the auxiliary winding current to lead the main winding current by angle Fields created by the two currents also have a phase difference of a thereby constituting an unbalanced field system. The result is the production of the starting torque
Slide24Resistance split 1
ph
I.M.
Slide25Application of Resistance split 1
ph
I.M.
It has a low starting current and moderate starting torque
.
It is used
for
easily
started loads and typical
applications.
fans, saws, grinders, blowers, centrifugal pumps,
office
equipment
, washing
machines etc.
available in the range of 1/20 to 1/2 kW.
Slide26Capacitor-start Motor
Slide27Capacitor-start Motor
It
uses the capacitor only for the purpose of starting
.
The
capacitor value
is
usually
so chosen as to give
a
= 90° elect
.
The
starting torque
is high.
capacitor need only be short-time rated
Slide28Capacitor-start Motor
Slide29Application of Capacitor-start Motor
Motor
has a high starting torque and therefore is used for hard starting
loads.
Typical application in compressors
, conveyors, pumps, certain machine tools, refrigeration and air-conditioning
equipment.
Available
up to sizes as large as 6 kW.
Slide30Two value capacitor motor
Slide31T
wo value capacitor motor
2-value capacitor motor not only uses a capacitor for starting but also
continuous
(run
) operation
.
The capacitor used permanently is called the
run capacitor,
the use of which improves
the
motor
running
performance.
At starting
(
both
capacitors
in circuit) where
>
90° elect. so that when the start-capacitor is disconnected becomes 90° elect.
Slide32T
wo value capacitor motor
Slide33Application of two value capacitor motor
It combines the advantages of capacitor-start and permanent-capacitor
motors and
is used for hard to start
loads.
It
gives a high power factor and efficiency under
running
conditions
.
applications
in
refrigerators, compressors and stockers.
Slide34Equivalent circuit parameter
A
single-phase induction
motor
can
to imagined to be consisting of two motors, having a common
stator winding
but with their respective rotors revolving in opposite
directions.
Each
rotor
has resistance and reactance half the actual rotor values
.
= resistance of stator
winding
= leakage reactance of stator
winding
X
= total magnetizing reactance
= resistance of the rotor referred to the stator
= leakage reactance of the rotor referred to the stator
Slide35Transformer eq.
ckt
of Single phase I.M
Rotor is stationary
Not showing forward and backward field rotor circuit
Slide36Rotating field equivalent
ckt.of
1-ph I.M with rotor stationary
Rotor stationary
Showing effect on equivalent
ckt
. by forward and backward field.
Slide37Rotating field equivalent
ckt.of
1-ph I.M with rotor running
Rotor is running
Showing effect on equivalent
ckt
. by forward and backward field.
Slide38No-load and blocked rotor test of 1 ph. I.M.
Aim to determine
the equivalent circuit parameters of a single phase induction motor by performing the
No-load and blocked rotor test .
Circuit diagram as:
Slide39No load Test
Motor is running at no load condition at rated voltage supply.
Motor consumes rated core loss and friction and
windage
loss.
Negligible
ohmic
loss.
=
No load rated voltage
=
No load stator current
=
No load losses.
No load test
=
/
reactance
+X
X=
=
No load
Test experimental procedure
C
ircuit
connections are made as per the circuit
diagram.
Variac
(auto transformer) is set to zero output voltage position before starting
the test.
Gradually increasing the voltage across the 1-
ph
I.M.
Motor is freely running at no load.
Variac
is varied slowly, until rated voltage is applied to
motor.
Reading of ammeter , voltmeter and wattmeter is taken when reaching rated voltage across 1-
ph
I.M.Variac is brought to zero output voltage position after test.Switch off the supply.
Slide42Blocked rotor test.
Rotor blocked condition (stationary)
Motor consumes
ohmic
losses
Core loss is negligible
Reading take place when rated current flowing.
=
Applied voltage
= I
nput rated current
=
Input power.
Blocked rotor test
=
/
Blocked rotor resistance,
Blocked rotor reactance,
=
=
/2;
=(
-
)